p53, the most frequently mutated tumor suppressor, is regarded as the guardian of the genome. Intensive studies over the past three decades have placed p53 in the center of a complex molecular network regulating diverse physiological responses to cancer- related stress signals. In previous studies, the components of the p53 network solely consist of protein coding genes, including those acting upstream to regulate p53 activity, those functioning downstream to mediate p53 effects, and those forming a regulatory feedback loop regulating the dynamics of the p53 activity. Recent studies from our group and others have identified a family microRNAs (miRNAs), mir-34a, b and c, as bona fide transcriptional targets of p53. These findings, for the first time, revealed the interplay between proteins and non-coding RNAs in this most important tumor suppressor pathway. miRNAs are a novel class of small, regulatory non-coding RNAs that mediate post-transcriptional gene silencing of a large number of target mRNAs. The ectopic expression of mir-34 miRNAs mimics the biological effects of p53 in growth arrest and apoptosis, possibly through their ability to dampen the expression of pro-proliferation and pro-survival genes. Given these preliminary findings, we hypothesize that the endogenous mir-34 miRNAs are key effectors to mediate many of the p53 downstream effects, and that the functions of mir-34 have profound impacts on tumorigenesis and tumor maintenance. Using cell culture studies and mouse genetic studies, we propose to determine whether mir-34 miRNAs mediate p53 induced growth arrest and apoptosis, and to what extent mir-34 miRNAs contribute to p53 mediated tumor suppression and tumor regression. PUBLIC HEALTH RELEVANCE: p53 is among the most important tumor suppressors identified, as indicated by its frequent mutations in human cancers. In response to a variety of cancer related stress signals, p53 can quickly accumulate inside the nucleus to activate and repress transcription of a number of targets, respectively. Through the functions of these downstream effectors, p53 exert anti-proliferative effects, induce apoptosis, block angiogenesis and promote DNA repair, which collectively prevent tumor formation and preclude tumor maintenance. A huge body of literature has been devoted to the key molecular events that regulate p53 activation or mediate p53 effects. However, most p53 research so far has focused on the protein components in the p53 pathway, which, unfortunately, neglected the potential impacts from non-coding RNAs, in particular, microRNAs (miRNAs). miRNAs exhibit abundance in the genome and possess great capacity for gene regulation and signaling transduction. In recent years, miRNAs, as a class of small regulatory RNAs, have emerged to be involved in nearly all of the essential cellular alterations associated with malignant transformation, including proliferation, replication potential, apoptosis, metastasis, and angiogenesis. It is clear that this family of genes is intimately integrated into the regulatory processes that are normally disrupted during transformation. Studies proposed here stem from our initial observations that mir-34 family miRNAs are direct targets of the p53 mediated transcription, and that enforced mir-34 expression recapitulate some of p53 functions. Our proposed studies presented here aim to elucidate the functions of mir-34 miRNAs in the p53 tumor suppressor network. The placement of these miRNA components into the well known p53 tumor suppressor network will help to elucidate the molecular basis and the molecular circuitry of the p53 pathway, and may also provide a paradigm for us to explore the functional cross talk between miRNAs and the oncogene and tumor suppressor network in general. Given the small size of miRNAs, these small RNA molecules can be easily delivered in vivo as a therapeutic agent. miRNAs have substantial potentials to be developed into diagnostic markers or therapeutical agents. Elucidating the roles of mir-34 family of miRNAs in the p53 pathway may also help us to evaluate the therapeutical potentials of this miRNA family, and may contribute to the cancer treatment and diagnosis in clinics.